* Do something with `src/attic/BoolEqs` and `src/attic/ChopFix`,
or remove them.
* Consider using two data fields in `node` instead of one,
so as to avoid using a separate `data` record. Benchmark.
* Provide an extensible-vector implementation of `IMPERATIVE_MAPS` for
integers? Like `ArraysAsImperativeMaps`, but does not require `n`.
* Provide an API in the style of Menhir's `FixSolver`, where constraints are
discovered incrementally during a first phase, then the solver is started?
* Develop a test suite. (Use `afl-fuzz`?)
E.g., in `CFG`, write a CFG generator.
Compare `Fix` with a naive solver.
* Develop a performance benchmark.